1. Field of the Invention
The present invention relates to a method for controlling the operational performance of gas discharge lamps in accordance with the preamble of claim 1 and also to a device for operating gas discharge lamps, in particular an electronic ballast, in accordance with the preamble of claim 5.
2. Description of the Related Art
FIG. 4 shows a known device for operating gas discharge lamps. This device, termed an electronic ballast, firstly comprises a rectifier arrangement 4 which converts an a.c. supply voltage into a rectified intermediate-circuit voltage which is fed to an inverter 5. The inverter 5 as a rule has two switches which are connected in series between a positive supply voltage and earth and which are activated alternately. The connection point between the two alternately activated switches, which are usually formed by MOS field-effect transistors, is connected to a load circuit which in the main contains a series-resonant circuit with a coil 6 and a capacitor 7 and also at least one gas discharge lamp 10. The gas discharge lamp 10 is connected to the series-resonant circuit by way of a coupling capacitor 8.
As a result of alternately switching the two switches of the inverter 5 on and off, the rectified intermediate-circuit voltage delivered by the rectifier 4 is converted into a high-frequency, switched-mode alternating voltage which is supplied by the inverter 5 to the series-resonant circuit. The gas discharge lamp 10 is fired in that the frequency of the alternating voltage delivered by the inverter 5 is shifted into the proximity of the resonant frequency of the series-resonant circuit having the coil 6 and the capacitor 7. In this case, a voltage overshoot occurs in the voltage applied to the capacitor 7 that results in the gas discharge lamp 10 being fired. In order to extend the life of the gas discharge lamp, moreover in FIG. 4 a filament-heating transformer 9A-C is provided, the primary winding 9A of which transformer is connected to the series-resonant circuit and is connected substantially in parallel with the gas discharge lamp 10 and the secondary windings 9B and 9C of which transformer are each connected in parallel with one of the two lamp filaments of the gas discharge lamp 10. The filament-heating transformer 9A-C is used to preheat the lamp filaments of the gas discharge lamp 10, with the filament or heating voltage having a frequency which lies clearly below or above the resonant frequency. In this way, a situation is avoided where the gas discharge lamp 10 fires with cold lamp filaments, whereby the life of the gas discharge lamp 10 can be extended. It is also possible to connect a heating capacitor, as an alternative to the filament-heating transformer 9A-C, in parallel with the gas discharge lamp 10. The use of a filament-heating transformer with secondary windings 9B and 9C connected to the lamp filaments of the gas discharge lamp 10 does, however, have the advantage that it is still possible to supply energy to the lamp filaments even after the gas discharge lamp 10 has been fired.
The series-resonant circuit, having the coil 6 and the capacitor 7, and also the gas discharge lamp 10 are part of a controlled system 2 which in turn is part of a control circuit, the performance of which is determined by a controller 1. In particular, the device shown in FIG. 4 is used to control the brightness of the gas discharge lamp 10 as a function of an externally predetermined desired dimming value SOLL which is compared with an actual dimming value IST in a comparator 3 formed as an adder, in which case the resultant differential signal DIFF is fed to the controller 1 which as a function of the system deviation DIFF generates a manipulated-variable value signal STELL for a specific controlled variable of the controlled system 2. In particular, the manipulated-variable value signal can relate to the frequency and/or the pulse duty factor of the switched-mode alternating voltage delivered by the inverter 5. In the case of the electronic ballast shown in FIG. 4, in order to determine the actual brightness value of the gas discharge lamp 10 a resistor 12 is provided that is connected in series with the lower lamp filament of the gas discharge lamp 10. The voltage dropping across the resistor 12 is a direct measure of the lamp current flowing by way of the gas discharge path of the gas discharge lamp 10 and the lamp current in turn is related directly to the degree of dimming or the brightness of the gas discharge lamp 10. Consequently, the actual value of the degree of dimming of the gas discharge lamp 10 can be acquired by determining the voltage dropping across the resistor 12.
The fundamental construction of the ballast shown in FIG. 4 is already known, for example, from DE 40 18 127 A1. It is proposed therein that the actual value of an operating variable of the electronic ballast be acquired, that the differential value between the acquired actual value and a predetermined desired value be formed and that this differential value be fed to a controller which as a function of the differential value generates a manipulated-variable value, for example for the alternating voltage of the inverter applied to the series-resonant circuit, in order, in this way, to control the lamp brightness of the gas discharge lamp activated by way of the series-resonant circuit.
On account of the series-resonant circuit, provided in the controlled system 2 shown in FIG. 4, that has the coil 6 and the capacitor 7, the controlled system 2 manifests in substance a PT.sub.2 characteristic (proportional response with second-order delay), i.e. the controlled system acts as a second-order delay element. It is known that controlled systems with a delayed P action require a PI controller in order to guarantee a transient response of the controlled system that is as rapid as possible and to counteract control-circuit instabilities which can develop as a result of an increasing tendency of the controlled system to oscillate. Advantageously, a PID controller is used in this connection, that is, a controller which has properties that both amplify in a proportional manner and also integrate and differentiate. Since, however, the structure of such a PID controller is very complex, for the sake of simplicity as a rule a PI controller is used as the controller 1 for the controlled system 2 shown in FIG. 4.
When the gas discharge lamp 10 is fired, the lamp current detected with the aid of the actual-value signal IST rises in an abrupt manner, whereby a light flash is generated in the gas discharge lamp 10, something which is to be avoided, however. It is therefore recommendable that in the case of a low level of brightness of the gas discharge lamp 10, that is, with a high degree of dimming, a controller 1 be selected, the proportional component of which is so high that it is possible to fire the gas discharge lamp 10 without a light flash even in the case of low brightness values. Such a high proportional component, that is, amplification of the controller 1, in the case of large lamp currents and a high lamp output, that is, with little dimming, would, however, result in instabilities in the control circuit within the controlled system 2 shown in FIG. 4.
A method for controlling the operational performance of gas discharge lamps is already known from DE 43 31 952 A1, wherein a parameter of the controller device used thereby is set as a function of a predetermined desired value. In particular, in accordance with this publication a controller device consisting of two blocks is used, with the one block forming the actual controller and the second block connected downstream of the first block forming a limiter which limits the output signal of the controller to a maximum value. An acquisition circuit arrangement is used to acquire the actual value of the lamp output of the gas discharge lamp and feed it to the controller which, moreover, receives a predetermined desired value and as a function of the difference between the actual value and the desired value generates a manipulated-variable value for the pulse duty factor of a switching controller of the electronic ballast. This manipulated-variable value is fed to the limiter which limits the manipulated-variable value in relation to an adjustable maximum value, in which case the maximum value of the limiter is set as a function of the predetermined desired value. In particular, the maximum value of the limiter is reduced in the case of a low desired value. The use of a controller with a proportional component and the problems connected therewith as previously explained are not, however, known from this publication.
A control method and an electronic ballast respectively are disclosed in EP-A1-0 605 052. A control circuit arrangement compares the lamp current, which occurs in the load circuit of the electronic ballast, with a predeterminable desired value and, as a function of the difference between the actual value of the lamp current and the desired value, generates a regulating signal for the frequency of the inverter. The control circuit arrangement has the control response of a PI controller in order to avoid instabilities. DE-A1-44 12 510 describes an illumination-control circuit arrangement having a feedback loop with a rate of response that can be changed over exclusively during the start-up phase. The rate of response is changed over as a function of the lamp voltage which is correlated with the lighting efficiency of the corresponding gas discharge lamp.